This relates to a method of calibrating conductive metal oxide electrodes and in particular to a method of setting the offset potential for such electrodes.
One problem which exists with all ion selective electrodes is that each electrode must be calibrated individually by determining an offset potential E.sub.0. For electrodes made of a combination of a metal and a thin metal oxide such as antimony/antimony oxide (Sb/Sb.sub.2 O.sub.3), palladium/palladium oxide (Pd/PdO) and iridium/iridium oxide (Ir/IrO.sub.2), the potential determining reaction involves protons such that the equilibrium equation between the metal and metal oxide has a form such as EQU IrO.sub.2 +4H.sup.+ +4e.revreaction.Ir+2H.sub.2 O. (1)
With respect to metal oxide electrodes wherein the metal is selected from the platinum or rhenium groups of metals such as those disclosed in application Ser. No. 441,902, the potential determining reaction involves two valence states of the hydrated oxide such that the equilibrium equation, for example, for thick iridium oxide has the form EQU IrO.sub.2 +H.sup.+ +e.revreaction.Ir(OH).sub.3 +H.sub.2 O. (2)
Because this chemical reaction is independent of the material on which the oxide is deposited, thick metal oxide electrodes can be coated onto metals other than the metal of the metal oxide or onto insulators as disclosed in application Ser. No. 441,902.
In the case of thick iridium oxide electrodes, the open circuit cell potential is specified by the equation EQU E=E.sub.0 +(kT/q) log.sub.10 [H.sup.+ ] (3)
where the offset voltage E.sub.O depends on the relative amounts of the hydrated iridium oxides of the two valence states. Thus, the output potential from a thick metal oxide of the platinum or rhenium groups depends on the concentration of hydrogen ions as is required in order to use such an electrode to sense pH. As will be apparent, the output potential also depends on the value of E.sub.0 which is the offset potential. In order to calibrate the electrode this value must be determined independently for each electrode, a time-consuming process.
In addition, the offset voltage E.sub.O is a time-dependent quantity when the electrode is kept in an open circuit, being dependent on the extent of hydration of the iridium oxide at the time of the offset voltage measurement. For example, if an open circuit electrode is immersed in an aqueous solution, E.sub.0 will move gradually with time, settling to a steady state potential of approximately 600 milliVolts vs. the potential of a saturated calomel reference electrode as shown in the data of FIG. 1. Such data is reproduced from T. Katsube et al. "pH Sensitive Sputtered Iridium Oxide Films," Sensors and Actuators, vol. 2, p. 399 (1982) which is incorporated herein by reference.